Method and apparatus for controlling breadown voltage in vacuum



May 21, 1968 M. RABINOWITZ 3,38 7

METHOD AND APPARATUS FOR CONTROLLING BREAKDOWN VOLTAGE IN VACUUM FiledOct. 22, 1965 2 Sheets-Sheet 1 FIG.5.

INVENTOR FIG.|. FIG.2. FIG.3.

May 21, 1968 Filed Oct. 22, 1965 M RABI NO WITZ METHOD AND APPARATUS FORCONTROLLING BREAKDOWN VOLTAGE IN YACUUM 2 Sheets-Sheet :v

| NVENTOR United States Patent 3,384,772 METHGD AND APIARATUS FORCONTROLLING BREAKDQWN VOLTAGE 1N VACUUM Mario Rabinowitz, 403 KingstonDrive, Wilkins Township, Pittsburgh, Pa. 15235 Filed Oct. 22, 1965, Ser.No. 501,782 21 Claims. (Cl. 313147) ABSTRACT OF THE DISCLOSURE Apparatusand a method for controlling the breakdown voltage between a pair ofspaced, electrically charged electrodes disposed within an evacuatedhousing are provided. The electrode faces are separated by a fixeddistance and at least one of the electrodes is shiftably mounted on thehousing so as to move relative to the other electrode without changingtheir separatiomMagnetic means is provided to establish a magnetic fieldof a given magnitude and direction in the space between the electrodes.The breakdown voltage is affected by changing the relative positions ofthe electrode faces since this changes the local heating of theelectrodes as well as gives rise to inertial forces on particles as theyleave the moving electrode faces. Also the breakdown voltage is affectedby the magnitude and direction of the magnetic field of the magneticmeans.

This invention relates to methods of increasing or decreasing thebreakdown voltage of electrodes in vacuum without changing theirseparation, and thus in particular by increasing the breakdown voltagewithout the usual decrease in electric field strength that has beenestablished between them.

The phenomena of electrical breakdown in vacuum is still not wellunderstood even though it was observed as early as 1897 by R. W. Wood,and has been the subject of many investigations since. The reason forthis may in part be due to the complexity of breakdown mechanisms whichmay all be at work in a given situation with one or more mechanismsdominating. In order to understand the way in which the breakdownvoltage may be controlled, i.e., increased or decreased, it is desirableto make a brief review of the various proposed vacuum breakdownmechanisms, and to define the term electrical breakdown in vacuum.

By increasing the potential between electrodes separated by vacuum, thecurrent between the electrodes steadily rises until a voltage is reachedat which the current suddenly increases by orders of magnitude. Acorrespondingly rapid decrease in the voltage between the electrodesalso occurs. The maximum voltage just prior to the voltage drop iscalled the breakdown voltage. The steep rise in current is accompaniedby a catastrophic spark. As used here, the terms electrical breakdown invacuum, vacuum breakdown, or more briefly breakdown, refer to thesephenomena. As used here, the term vacuum denotes pressure less than 10-torr, and preferably less than l0 torr; or more generally that the meanpath between charged particle and neutral gas particle collisions isgreater than the electrode separation.

In 1952, L. Cranberg suggested that breakdown is initiated when acharged clump of material is removed from one electrode surface underthe influence of the electric field, strikes the opposite electrode, andthus causes suffciently high temperature to produce local evaporation.This led him to conclude that for plane parallel electrodes thebreakdown voltage, V, and the electrode gap length, d are related thus:V=Kd where K is a constant characteristic of the electrodes. In 1957, I.N. Slivkov modified this hypothesis slightly, and derived the relation:V=.Kd for plane-parallel electrodes.

The next hypothesis postulates that evaporation of the anode is producedby bombardment of an electron beam issuing from the cathode, which leadsto breakdown in the anode vapor. A number of people are associated withthis hypothesis, some of whom are L. B. Snoddy (1931), I. W. Beams(1933), and J. A. Chiles (1937). More recently A. Maitland in 1961derived an equation from this hypothesis of the form V Kd. He gives anequation for a in terms of the electric field and the gap, d.

In 1936, A. J. Ahearn proposed the following model of local heating ofthe cathode to explain initiation of breakdown. Before breakdown, thefield emission currents come primarily from a few of the microscopiccathode surface projections, where the local field is intensified bytheir presence. In addition the force on the cathode surface due to theelectric field is greatest at these projections. There is localresistive heating, which depends on the size and geometry of theseprotrusions, as well as their thermal contact with the body of thecathode. As the electric field is increased sufiiciently, a ruptureoccurs at the protrusion where conditions of electric force, resistiveheating, and tensile strength are most favorable. This rupture then canlead to breakdown. Modified versions of this model have also beenproposed.

The basic assumption of the next hypothesis is that at a criticalvoltage a free, charged particle upon striking an electrode produces anavalanche of charged particles by secondary emission; with photoemission also playing a role. For example electrons striking the anoderelease positive ions which in turn release electrons upon striking thecathode, etc. In 1947 J. G. Trump and R. J. Van de Graatf proposed thepositive ion avalanche hypothesis to account for the inititiation ofvacuum breakdown. In 1948, J. L. McKibben and R. K. Beauchamp proposedthe positive ion-negative ion avalanche hypothesis.

In 1963, W. D. Owen and F. Llewellyn-J ones proposed the hypothesis thatvacuum breakdown results from an ordinary Townsend discharge in the gasliberated between the electrodes from the gas adsorbed on the electrodesurface.

As you can see there are a multiplicity of hypotheses to account forvacuum breakdown, each of which is based on a difierent model of theinitiating mechanism. Though each hypothesis may be partly correct, myexperimental investigation has shown that no hypothesis can account forall the experimental facts. This has led me to introduce a newhypothesis which is capable of predicting at least qualitatively many ofthe experimental results not predicted by previous hypotheses.

My hypothesis provides a simple concept that can predict at leastqualitatively the known experimental results. No particular model interms of processes that are assumed to occur is associated with thishypothesis. Rather, it is concerned with the energetics of the problem.The initiation of breakdown and gap conduction can be extremely rapid.The voltage drop can occur in the order of nanoseconds which is so fastcompared to the time constants of most breakdown circuits that only thecapacitively stored energy of the electrodes and supports dischargeswithin this time. After breakdown has occurred, most of the energy ofthe power supply is dissipated elsewhere in the circuit because the arcvoltage is so low compared to the voltage drops across the other circuitcomponents.

Hence it is reasonable to assume that the energy available to initiateelectrical breakdown in vacuum is equal to some traction of thecapacitively stored energy of the electrodes. If the available energy isless than the needed energy, breakdown cannot occur. It is the object ofthis invention to control breakdown by controlling the efficiency of thebreakdown process, or the fraction of the energy available to initiatebreakdown. As described in my paper in Vacuum, 15 (1965), 59, for theplane parallel electrodes this hypothesis leads to the equation:

where V is the breakdown voltage; W is the energy needed to initiatebreakdown; s is the permittivity of vacuum; A is the area of anelectrode; d is the distance between the electrodes or gap length; and fis the fraction of the capacitively stored energy avail-abie to initiatebreakdown. From this equation we see that by changing f we can changethe breakdown voltage, either increasing or decreasing it.

For a better understanding of my invention, reference may be had to thefollowing drawings taken in conjunction with the accompanyingdescriptions wherein:

FIG. 1 is a fragmentary perspective view of two electrodes rotating inthe same direction.

FIG. 2 is a fragmentary view in axial cross-section of two rotatingelectrodes showing the configuration of the magnetic field which isproduced by their rotation.

FIG. 3 is a fragmentary sectional view of two rotating electrodeswherein the two axes of rotation do not coincide.

FIG. 4 is a fragmentary perspective View of two electrodes rotating inopposite directions about the same axis showing particles leaving theelectrodes due to inertial forces.

FIG. 5 is a view in axial cross-section of two electrodes in a vacuumchamber showing a liquid metal seal of the electrode supports at thechamber walls.

FIG. 6 is a fragmentary cross sectional enlarged View of an electrodeand support, showing in detail a liquid metal seal.

FIG. 7 is a fragmentary view in axial cross-section showing anothermethod for rotating the electrode by means of a magnetic coupling of theshaft inside the chamber with a magnetic drive outside the chamber.

FIG. 8 is a sectional view of two electrodes in a vacuum chamberillustrating a reciprocating bellows action for oscillational motion ofthe electrodes.

Whereas particular embodiments of my invention are shown and described,it is to be understood that various changes, modifications, andalternative construct-ions may be employed without departing from thetrue spirit and scope of my invention.

An important point to bear in mind is that if the breakdown voltage ofgiven conditioned electrodes inside a vacuum chamber is increased byincreasing their gap separation, the electric field between them isdecreased since the breakdown voltage varies as the gap distance by apower less than 1, V=Kd 0 a 1; and the electric field is, E=V/d. Myinvention allows one to increase or decrease the breakdown voltagewithout changing the electrode gap. Hence for a given gap :a largerelectric field could be produced than could otherwise be produced andmaintained.

The previously described hypotheses suggest that if there were arelative motion between anode and cathode, the breakdown voltage wouldbe increased. This relative motion and the ensuing effects are theessence of this invention. For example, local heating of the anode by anelectron beam coming from the cathode would thus occur at continuallychanging fresh surface :areas of the anode rather than be confined to astationary spot. This would result in less heating at any given spot.The relative motion, without changing the electrode separation could beachieved in a number of ways. For example the electrodes could berotated in opposite directions around a common axis. Or they could berotated in the same direction, with their axes displaced as in FIG. 3.Even rotating only one electrode or the other would be etfective in thisease. Relative motion could also he achieved by a re ciprocal sidewaysoscillation of one or both of the electrodes; or by other means. In thisexample, the electron beam does not focus on the same electrode area,and similarly neutral and charged particles released from the anode donot move back toward the electron emission site on the cathoderesponsible for their release thus decreasing mutual interaction betweenactive anode and cathode sites leading to an increase in breakdownvoltage.

Relative motion as described in the preceding paragraph can be effectiveeven when mechanisms other than electron beam heating of the anode areimportant. The relative motion of the electrodes gives rise to inertialforces on particles as they leave the moving electrode surface. Thistends to either throw them out of the electrode gap and miss theopposite electrode or to strike the opposite electrode at more of aglancing angle and hence be less effective than otherwise. That particleinertia effects can occur and are present to a significant degree isshown in the photographs of FIGS. 10, 11 and 12 of my paper in Vacuum,15 (1965), 59.

Since electrical discharges in vacuum are essentially supported byvaporized metal vapor atoms and other particles removed from theelectrode surfaces, any radial motion of the electrodes perpendicular tothe electrode axes will be imparted to these particles as they leave theelectrode surfaces. For example, if a particle leaves a moving electrodewith a radial velocity 1 its trajectory in a uniform axial electricfield E will be parabolic. if the particle leaves an electrode rotatingwith angular velocity, 0:, at a radial distance r, v=wr. The particlewill be radially displaced a distance,

21nd 1/2 i: a

where m is the particle mass, q is the charge on the particle, d is thegap separation, and E is the electric field. This may cause it to eithermiss the opposite electrode, or to hit it at a glancing angle. When theparticle hits the opposite electrode its momentum vector makes an anglewith respect to the electrode axis, making it less effective in knockingout secondary particles, in sending them in the direction of theelectrode axis, and in returning them to the particles original site. Vis the voltage between the electrodes. If the particle is not charged,it will move radially out of the electrode gap, neglecting polarizationeffects.

In addition to the obvious effects of changing areas which face eachother, an inertial forces on particles as they leave the electrodesurfaces, there is also the more subtle effect that a magnetic field isgenerated by the charge distribution on the moving electrode, whichfurther complicates the situation. This method of producing a magneticfield between the electrodes circumvents the necessity of either usingelectromagnet coils near the electrodes with the associated currentcarrying wires that must be brought in, or of using permanent magnetsnear the electrodes. If the electrodes are to be baked out at hightemperatures both the electromagnet coils and the permanent magnetsplaced just behind the electrode surfaces would present limitationswhich the production of the magnetic field by motion of the chargedelectrode would not be vulnerable to. There may well be additionalreasons where a magnetic field is desired between the charged electrodesand where one can not use permanent magnets or electromagnet coils. Inthose situations where conditions permit, permanent magnets and/orelectromagnets may be used next to the electrode faces to increase orchange the magnetic field between the electrodes.

Since the electrodes are oppositely charged, rotating them in oppositedirections causes the fields to add together and produce an almostuniform magnetic field, While rotating them in the same direction willproduce an almost radial magnetic field. For example, consider a planecircular disk electrode of uniform surface charge density, rotating withangular velocity, to. In cylindrical coordinates, p, z, and 15, thecomponents of the magnetic flux density, B, are given by:

w is the angular velocity of the disk in radians/sec;

R is the radius of the disk in meters;

0 is the surface charge density in coulombs/meter ,u is approximatelythe permeability of the electrode material at regions near theelectrode, otherwise it is the permeability of free space inhenrys/meter; and p and z are measured in meters. The origin is at thecenter of the disk and the disk is perpendicular to the z-axis. A and Gare complete elliptic integrals of the first and second kind:

From these equations for B we see that when two such electrodes ofopposite charge, separated by a distance d, rotate in the samedirection, the axial fields l3 almost cancel, while the radial fields B,add together. When electrodes of opposite charge rotate in oppositedirections the axial field B add, while the radial fields B, almostcancel.

So for electrodes with an electric field, E, between them and rotatingin the same direction about the same axis, B is approximately zero andB, is proportional to Ewe p. where E is measured in volts/ meter, and sis the permittivity of free space in coulomb /newton-meter In this casethe electric and magnetic fields are essentially perpendicular to eachother which can lead to an increase in breakdown voltage.

For electrodes with an electric field, E, between them and rotating inopposite directions about the same axis, B is approximately zero and Bis proportional to Ewe In this case the electric and magnetic fields areessentially parallel or antiparallel to each other which can lead to adecrease in breakdown voltage as in a Penning discharge. However, byusing permanent magnets, and/or electromagnets, just behind theelectrode faces, with like poles opposite, the magnetic field betweenthe electrodes can be made essentially radial even when the electrodesrotate in opposite directions leading to an increase in breakdownvoltage. Conversely if the permanent magnets or electromagnets areplaced just behind the electrode faces, with unlike poles opposite, theresulting magnetic field will be increased axially.

The equation of motion of a charged particle in crossed electric andmagnetic fields is given in vector notation by d A A m= =qE+qvAB Themotion is complicated and depends on the initial velocity. Qualitativelyone can say that when the fields are perpendicular the charged particlemoves in a cycloidal like .path with net displacement perpendicular to Eand B, and when they are parallel the path is helical with netdisplacement parallel to 1 5 and The effect of the magnetic field canusually be neglected when the magnetic flux density is small. This needsto be determined in each situation by the angular or linear velocity ofthe electrodes and the other variables which give the magnetic fluxdensity in relation to the electric field. The magnetic field in the gapregion can be increased by using electrodes of high permeabilitymaterial such as iron; or by placing a slug of such material immediatelybehind each electrode face or by placing permanent magnets orelectromagnets immediately behind each electrode face.

The relative motion of high voltage electrodes in vacuum can bebeneficial to a great variety of devices. The following list isillustrative, but does not represent all such devices: high energyparticle separators used in connection with accelerators; low loss highfrequency vacuum capacitors; high intensity pulsed over-voltage X-raytubes; vacuum electronic tubes; high energy particle accelerators;photo-multiplier tubes; microwave tubes such as klystrons; controllednuclear fusion devices; ion propulsion engines; electrostaticvoltmeters; outer space electrostatic shielding against high energycharged particles; and vacuum switches.

For example in the case of vacuum switches, the relative motion of theelectrodes can be beneficial during arcing as well as to withstand ahigher voltage following arcing. It is important in vacuum switches toavoid undue electrode erosion. In accordance with this invention thisobjective is accomplished by the relative motion of the electrodes. Thismotion continually presents fresh electrode surfaces to the arc andhelps to prevent pocking erosion at any one point on the electrodes. Inaddition the tangential velocity imparted to electrode particles withensuing inertial effects as they enter the arc helps to improve thearcing characteristics and the extinguish the arc.

The novel features which I believe to be characteristic of my presentinvention are set forth with particularity in the appended claims. Myinvention itself, however, both as to its organization and method ofoperation, together with further objects and advantages thereof, will bebetter understood by reference to the following descriptions taken inconnection with the accompanying drawings which represent particularembodiments of my invention.

Referring now to FIG. 1, there are shown two axially spaced coaxialelectrodes 1a and 1c embedded in a vacuum chamber or the vacuum of outerspace. The electrodes 1a and 1c rotate with angular velocities ca and wrespectively about their common axis 2 shown as a dotdash line. There isa slight depression 3c at the center of electrode 1c, and a similardepression 3a at the center of electrode 1a to avoid axial effects,i.e., to insure that a potential breakdown site will occur otf-axis. Theelectrodes 1a and 10 as shown are of cylindrical conformation about axis2; however they may also have other shapes. As shown, the electrodes arerotating in the same direction. This will bring into being inertialeffects, as well as a radial magnetic field as shown in FIG. 2. Howeverthis invention may be practiced by the rotation of only one of the twoelectrodes, or by the rotation of both electrodes in opposite directionsas will be described in connection with FIGS. 3 and 4.-The electrodesare supported, respectively, by the conducting support arms 4a and 4c.

As shown in the axial cross-sectional view of FIG. 2, two electrodes 1aand 1c, in vacuum, are rotating in the same direction around theircommon axis 2. There is a slight depression 3a and Scat the center ofeach electrode to insure that breakdown will not occur on axis. Theelectrodes are supported by the conducting supporting arms 4a and 4c towhich the power source is connected. Shown embedded immediately behindthe face of each electrode 1a and 1c are permanent magnets 5a and 5c toenhance the radial magnetic field. Of course electromagnets could beused in place of the permanent magnets. As shown, these magnets 5a, Sohave like poles facing opposite each other to produce a radial magneticfield. If an axial magnetic field is desired then unlike poles would beput facing each other. In situations where neither permanent magnets norelectromagnets could be used, a radial magnetic field would still beproduced by the rotation in the same direction of the two chargedelectrodes 1a, 10. This magnetic field could be enhanced by placinginserts of high permeability material such as soft iron or silicon .eelimmediately behind the face of each electrode in, 1c in the positionshown occupied by the permanent magnets Sa, 50. Alternatively, theelectrodes la and la themselves can be made of a high permeabilitymaterial. The resultant magnetic field distribution is indicatedschematically by the three representative magnetic flux lines shown asdashed lines. There are of course many such flux lines making up theoverall field.

As shown in FIG. 2 the magnetic field fringes radially outward. In apreferred form of this invention, the faces of the electrodes and 1c areslightly convex with depressions 3a and 3c at the center so that theyare in accordance with the shape of the magnetic flux lines andsubstantially parallel to adjacent fiux lines. Since the electric fieldlines are perpendicular to the electrode faces, this helps to make theelectric field lines and magnetic field lines mutually perpendicular andassures maximum interaction of a charged particle with the fields.However the electrode faces may also be plane or any other shape. Inaddition to the magnetic field action, both neutral and chargedparticles acquire a radial velocity as they leave the electrode face dueto the rotation of the electrodes. The benefits from these effectsleading to an increased breakdown voltage have previously beendescribed.

FIGURE 3 shows a sectional view of two electrodes 1a and 1c, in vacuum,rotating in opposite directions about their respective axes 2a and 2cwhich are displaced radially from each other. This displacement helps tominimize the effects of the magnetic field that is produced. Theelectrodes are supported, respectively, by the conducting support arms4a and 46. Since the angular velocities w, and w are in oppositedirections, both the inertial effect and the effect of continuouslychanging electrode areas are brought into play. It should be noted thateven if the electrodes rotate in the same direction about theirrespective axes, given locations on one electrode, face oppositecontinuously changing locations on the opposite electrode because theaxes of rotation do not coincide.

In FIG. 4, the electrodes 1a and 1c, in vacuum, are rotating in oppositedirections about their common axis 2. There are depressions 3a and 30,respectively at the center of each electrode 1a and 1c to insure thatpotential breakdown sites will occur ofit axis. The electrodes in and 1care supported respectively by the conducting support arms 4a and 4c. Twoparticles 6a and 6e are shown respectively leaving the electrodes 1a and10 due to inertial forces arising from the motion of the electrodesimparted to the particles as they leave the electrode surfaces. Therotation of the electrodes in opposie directions causes given locationson one electrode to face opposite continuously changing locations on theopposite electrode, leading to an increase in breakdown voltage. Eventhough the rotation of the electrodes in opposite directions tends toproduce a magnetic field in the axial direction parallel to the electricfield which would decrease the breakdown voltage, by proper choice ofgeometry (electrodes radius, depression size, spacing, etc.), chargedensity, angular velocity, and other factors as previously discussedthis magnetic field can be made very small so that its effects arenegligible and the breakdown voltage increases due to changing area andinertial effects. And as previously described, permanent magnets and/ orelectromagnets can be placed behind the electrode faces to make themagnetic field either essentially radial or essentially axialindependent of the relative motionof the electrodes. A radial magneticfield would help to further increase the breakdown voltage.

Referring now to FIG. 5, there are shown two axially spaced coaxialelectrodes 1a and 1c embedded in at hermetically sealed vacuum chamber.As shown, electrodes 1a and 1c rotate in opposite directions withangular velocities w, and w about their common axis 2. There aredepressions at 3a and to insure that breakdown cannot occur near theaxis 2. The electrodes in and 1c are supported, respectively, by theconducting support arms, 4:: and 4c. Wound respectively around thehollow supporting arms 4a and 4c, and just behind the electrodes 1a and1c are two electromagnet coils 7a and 7c which serve the same purpose asthe permanent magnets of FIG 2, but which add versatility to the system.When the fields of the two electromagnet's produce like poles oppositeeach other, an essentially radial magnetic field is produced between theelectrodes as shown in FIG. 2, which helps to increase the breakdownvoltage. When the fields of the two electromagnets produce unlike polesopposite each other, an essentially axial magnetic field is producedbetween the electrodes which tends to decrease the breakdown voltage.The effects of the electromagnets 7a, 7c are independent of the motionof the electrodes 1a, 10, i.e., these effects are present whether or notthe electrodes 1a and 1c are in motion. The effects of the fieldsproduced by the electromagnets 7a, 7e superimpose on the motionaleffects of the electrodes la, lie when both are present. The current tothe electromagnets 5a and 5c is brought in by the leads 8a and throughthe hollow support arms 4a and 40 so as not to interfere with therotation of the electrodes 1a, 10. This may also be accomplished by theuse of a commutator or slip ring arrangement.

As shown in FIG. 5, the electromagnet 7a and 7c are respectively poweredby the power sources 9a and 90. As shown these power sources 9a and 9care direct current, but for some purposes they may be alternatingcurrent. The power source 9a is turned on and off by the simple switch10a. The power source is turned on and off by the double pole doublethrow switch Title, which also permits changing the polarity of thepower source 9c and hence the polarity of the magnetic field produced bythe electromagnet 70. This makes it easily possible to change from aradial magnetic field to an axial magnetic field or vice versapermitting one to readily decrease or increase the breakdown voltage. Ofcourse the combination of one electromagnet and one permanent magnetbehind each electrode in and 1c can also serve this purpose. Howeverthis does not allow for as much flexibility as when two electromagnets7a, 70 are used, wherein both the magnitude and the direction of theapplied magnetic field between the electrodes 1a and 10 may be easilychanged by varying the power supplies 9a and 9c accordingly. If theswitches 10a and are left open, there will be no applied magneticfields, and the effects are due to the motion of the electrodes 1a and10. Alternatively, the electrodes may be stationary with the effectsentirely due to the applied magnetic fields. Two electric lines L and Lmake electrical connection by slip rings or similar method to thesupporting arms 4a and 4c to provide high voltage to the electrodes 1aand 10.

Also shown in FIG. 5, are the other components of the hermeticallysealed vacuum chamber, including the dielectric insulating wall 11, withsealed on end flanges 12a and 120. A liquid metal seal 13a and 13c inmade with each supporting arm 4a and 4c at the flanges 12a and 120. Thisseal, which is described in greater detail in connection with FIG. 6,permits both rotation and axial motion of the support arms 4a and 40 towhich the electrodes 1a and 1c are attached. Such seals would not benecessary if the motion producing devices were inside the vacuumchamber. A vacuum chamber with seals for rotation and axial motion wouldnot be needed if the electrodes la, 10 were in the vacuum of outerspace.

FIGURE 6 shows an enlarged view in detail of the liquid metal sealmentioned in connection with FIG. 5. There is shown the electrode 1,rotating about the axis 2, supported by the supporting arm 4. A liquidmetal 20 provides the seal between the support arm 4, and the flangewall 12. Preferably the liquid metal is one of low vapor pressure, lowmelting point, wets the metal surfaces at the seal, and isnon-corrosive. A liquid metal alloy made of 62.5% gallium, 21.5% indium,and 16% tin with melting point of about 10 C. and vapor pressure lessthan 9 10- torr up to 500 C. can be used for this purpose even though itis corrosive at elevated temperatures. To prevent corrosion, thesupporting arm 4 may have a shaft clad 21, and the wall may have aninsert 22 that are resistant to corrosion by the liquid metal 20. In thecase of the gallium-indium-tin alloy just described, the shaft clad 21and the insert 22 may be made of tungsten or tantalum for hightemperature use, or stainless steel where the main use is at roomtemperature. There is approximately a 0.005 inch gap all around for theliquid metal between the shaft clad 21 and the hole in the insert 22, Onthe atmospheric side of the flange 12, a forepump vacuum of about 10torr is maintained to reduce the pressure differential on the liquidmetal 20. Inside the enclosure 23 the pressure is 10 torr. Thisenclosure 23 may be sealed with rubber O-rings 24 as shown. Inside theenclosure 23 are ball bearing races 25 to hold the supporting arm 4 inalignment to permit it to rotate freely.

FIGURE 7 shows an alternate means for providing rotation of theelectrodes. There is shown the electrode 1 rotating about the axis 2,and supported by the supporting arm 4. A permanent magnet 3% is attachedto the end of the support arm, perpendicular to axis 2 and parallel tothe flange. The support arm 4 and the magnet are held by the sleeve 31.Ball bearing races 32 hold the support arm 4 in alignment and permit itto rotate freely. An electric line L is connected to the outside of thesleeve 31 to provide voltage to the electrode 1. An external magnet 33couples its rotational motion to the internal magnet 30. The same effectcan be achieved by several fixed electromagnets with oscillating fieldsas is done by the field coils in some motors.

FIGURE 8 shows a way in which linear oscillational motion may beimparted to two electrodes 41a and 410 in a hermetically sealed vacuumchamber 40. The electrodes 41a and 41c oscillate back and forth in thedirections respectively indicated by the arrow tipped lines 42a and 420in the plane of the paper, which motion is permitted by thereciprocating members 43a and 430 shown as bellows. The electrodes 41aand 410 are supported respectively by the conducting support arms 44aand 440 which are attached to the bellows 43a and 43c. The bellows 43a,43c are attached to the end flanges 45a and 450. The bellows 43:1, 430allow for a bending motion in addition to the linear motion, if desired.The end flanges 45a, 45c are separated by an insulating wall 46.Electric lines L and L are attached to the support arms 44a and 440 toprovide voltage to the electrodes 41a and 410.

In FIG. 8 the linear motion of the electrodes 41a, 410 in oppositedirections relative to each other causes given locations on oneelectrode to face opposite continuously changing locations on theopposite electrode. The motion of the electrodes 41a, 41c, also impartsa velocity perpendicular to the electric field as particles leave theelectrodes which results in the previously discussed inertial effects.The linear motion of the charged electrodes 41a, 410 also generates amagnetic field between the electrodes which can be made negligible byproper choice of parameters similar to the rotational case. As before, amagnetic field may be applied, if desired, by placing electromagnets orpermanent magnets immediately behind the electrode faces 41a and 41c. Asbefore, when the like poles are placed opposite each other anessentially radial magnetic field perpendicular to the electric field isproduced between the elcctrodes 41a, 410 which acts to increase thebreakdown voltage. When opposite poles are placed opposite cach other, amagnetic field is produced between the electrodes 41a, 41c essentiallyparallel to the electric field which acts to decrease the breakdownvoltage. Another mode of oscillation is in which the two electrodes 41a,41c oscillate in phase so they move together back and forth in the samedirection at the same time. In this case, the changing area effect iseliminated as the same locations would continue to face opposite eachother on the two electrodes 41a, 410. However the inertial and magneticeffects may still be present.

While I have shown and described specific forms of the present inventionit will of course be understood that many modifications'and alternativeconstructions can be made without departing from its spirit and scope.I, therefore, intend by the appended claims to cover all suchmodifications and alternative constructions as fall within their truespirit and scope.

What I claim as new and desire to protect by Letters Patent of theUnited States is:

l1. An electronic device comprising: an evacuated housing; a pair ofelectrodes; means mounting said electrodes at fixed, spaced apartlocations within said housing and including structure permitting atleast one of the electrodes to shift along a preselected path relativeto the housing as the spacing between said electrodes is held constant,whereby the electrodes are movable relative to each other; means coupledwith the electrodes for establishing an electrical field therebetween,having a predetermined value whereby the electrodes will be oppositelycharged; and means coupled with said one electrode for shifting the samerelative to the other electrode along said preselected path at a ratesutficient to control initiation of electrical discharge between saidelectrodes when said electric field is maintained at said value.

2. An electronic device as set forth in claim 1, wherein said oneelectrode is rotatably mounted on said housing.

3. An electronic device as set forth in claim 18, wherein said oneelectrode is mounted for rectilinear movement on said housing.

4. An electronic device as set forth in claim 1, wherein said housinghas a pair of opposed walls, each electrode having a support armrotatably mounted on a respective wall and held against axial movement,said electrodes being secured to the inner ends of respective arms, saidshifting means being coupl d to said arms for rotating the same.

5. An electronic device as set forth in claim 4, wherein said shiftingmeans includes structure for rotating said arms in the same direction.

6. An electronic device as set forth in claim 4, wherein said shiftingmeans includes structure for rotating the arms in opposite directions.

7. An electronic device as set forth in claim 4, wherein said arms arein axial alignment, said electrodes having opposed faces, each facehaving a central depression, the depressions being in alignment witheach other.

8. An electronic device as set forth in claim 4, wherein said electrodeshave opposed faces, said arms being parallel and axially offset tothereby ofiset said opposed faces from each other.

9. An electronic device as set forth in claim 4, wherein is providedmeans coupled With said electrodes for establishing a magnetic field inthe space therebetween.

10. An electronic device as set forth in claim 9, wherein saidestablishing means includes a permanent magnet for each electroderespectively, the magnets being within said housing and adjacent torespective electrodes.

11. An electronic device as set forth in claim 9, wherein saidestablishing means includes an electromagnet for each electroderespectively, the electromagnets being within said housing and adjacentto respective electrodes, and electrical power means coupled with eachelectromagnet for actuating the same.

12. An electronic device as set forth in claim 11, wherein said powermeans for one of said electromagnets includes a circuit having a currentreversing switch, and means for varying the electrical power supplied tosaid electromagnet.

13. An electronic device comprising an evacuated housing; a pair ofelectrodes, each electrode having a face; means mounting said electrodeswithin said housing with said faces being separated by a predetermineddistance and held against movement toward and away from each other;means coupled with said electrodes for establishing an electric fieldtherebetween having a predetermined value, whereby the electrodes areoppositely charged; means coupled with said electrodes for establishinga magnetic field therebetween having a magnitude and direction tocontrol the initiation of electrical discharge between the electrodes assaid electric field is maintained at said predetermined value.

14. An electronic device as set forth in claim 13, wherein saidestablishing means includes an electromagnet for each electroderespectively, the electromagnets being within said housing and adjacentto respective electrodes, and an electrical power source for eachelectromagnet respectively, one of said power sources including acircuit having a reversing switch and means for varying the electricalpower to the respective electromagnet.

15. A method of controlling the breakdown voltage between a pair ofelectrodes spaced a fixed distance apart in a vacuum comprising thesteps of: coupling said electrodes to a source of electrical power tooppositely charge the electrodes and thereby to establish an electricalfield therebetween having a predetermined value; and moving a path andat a rate sutficient to control the initiation of a path and at a ratesufficient to control the iniiaion of electrical discharge between theelectrodes .as said electrodes are maintained separated by said fixeddistance and as said electric field is held at said predetermined value.

1 6. A method as set forth in claim 15, wherein said moving stepincludes rotating the electrodes in the same direction.

17. A method as set forth in claim 15, wherein said moving step includesrotating the electrodes in opposite directions.

18. A method as set forth in claim 15, wherein said moving step includesreciprocating said electrodes along respective, rectilinear paths.

19. A method as set forth in claim 15, wherein is included the step ofestablishing a magnetic field between the electrodes.

20. A method of controlling the breakdown voltage between a pair ofelectrodes spaced a fixed distance apart in a vacuum comprising thesteps of: coupling said electrodes to a source of electrical power tooppositely charge the electrodes and thereby to establish an electricalfield therebetween having a predetermined value; generating andmaintaining -a magnetic field in the space between the electrodes withthe field having a magnitude and direction sufficient to control theinitiation of electrical discharge between the electrodes as saidelectrodes are maintained separated by said fixed distance and as saidelectric field is held at said predetermined value.

21. A method as set forth in claim 20, wherein said magnetic field has acomponent for each electrode respectively, with each component having apreselected polarity, and wherein is included the steps of selectivelyreversing the polarity of one of said components, and changing themagnetic field intensity of a first of said components.

References Cited UNITED STATES PATENTS 660,613 10/1900 Baker 313-1492,116,218 5/1938 Seil 313-149 2,562,031 7/1951 Gerber 313-149 2,712,0976/1955 Auwarter 313- X 2,793,325 5/1957 Wenzel 313-149 X 2,831,9944/1958 Schering 313-149 X 2,831,995 4/1958 Agule 313-149 X 2,985,7885/1961 Opsahl 313-153 X 3,020,408 2/ 1962 Martin 313-61 X 3,082,307 3/1963 Greenwood 313-153 X 3,097,292 7/ 1963 Kugler et al. 219-1213,320,462 5/1967 K-awiecki 313-154 DAVID J. GALVIN, Primary Examiner.

STANLEY D. SCHLOSSER, JAMES W. LAWRENCE,

Examiners. R. L. J UDD, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,384,772 May 21, 1968 Mario Rabinowitz It is hereby certified thaterror appears in the above numbered patent requiring correction and thatthe said Letters Patent should read as corrected below.

In the heading to the printed specification, lines 4 and 5, for "403Kingston Drive, Wilkins Township, Pittsburgh, Pa. 15235" read Station A,P. O. Box 695, Menlo Park, Calif. 94025 column 3, line 4, the equationshould appear as shown below instead of as in the patent:

column 4, line 49, for "an" read and column 5, line for "field" readfields line 60, the equation should appear as shown below instead of asin the patent:

column 6, line 27, for "the", second occurrence, read to column 7, line50, for "opposie" read opposite line 57, for "electrodes" read electrodecolumn 8, line 56, for "in" read is column 9, line. l8,cafter"alignment" insert and ;column 10, line 29, for the claim reference,numeral "18" read 1 column 11, line 22,

after "moving" insert one of the electrodes relative to the otherelectrode along Signed and sealed this 31st day of December 1968.

[SEAL] \ttest EDWARD M.FLETCHER,JR. EDWARD J. BRENNER \ttesting OfficerCommissioner of Patents

